Over the years, we all are faced with decisions like this and we must look at each opportunity with fresh eyes as the decisions I made in Year 1 would be much different in Year 5 as each opportunity will be different.  

You must decide if the opportunity would be one that you would pursue without this potential collaboration or not.  Is it something you would consider hiring a firm to JV with or is the labor and material cost too high to commit the company to with your existing clients' workload already
contracted and commencing.  Weighing options is a great problem to have and all should be considered based on your business' existing workload, personnel, and goals.

The last thing anyone should do is over-commit and under-deliver.  Take care of existing clients' needs first and then respond to the request to collaborate.  If you start the collaboration and don't take care of your clients who have been loyal to you, you risk your entire company's future if you mismanage the opportunity for positive growth.  

From a owner's perspective, it is not just a manpower/staffing issue, but a responsible and positive, managed growth issue.

Your first responsibility is to your existing clients and everything else comes after that.